In certain circumstances, it is desirable to inject medication directly into human tissue. Typically, syringes are used to inject medicaments into tissue areas, such as the intramuscular tissue layer, the subcutaneous tissue layer, and the intradermal tissue layer. Each of these tissue layers has specific characteristics that affect the amount of fluid pressure needed to inject a fluid into the targeted tissue layer. When injecting fluids into each of these tissue layers, the user must exert enough force on the injection device to overcome different amounts of backpressure associated with the particular tissue layer. In general, practitioners and self-injectors, such as diabetics, are familiar with the force necessary to inject fluids into the subcutaneous layer. Injections into the subcutaneous and intramuscular tissue layers can cause discomfort to the patient or self-injector because of the characteristics of the tissue, needle length and needle diameter or gauge. It is desirable to employ shorter, smaller gauge needles to achieve delivery into the intradermal tissue layer.
It is noted that when the needle lengths are shortened and needle diameters are made smaller, the fluid dynamics of the injection device changes. Additionally, the fluid dynamics between the injection device and the targeted tissue layer also change because the shorter needle length injects the fluid into a different tissue layer, such as the intradermal layer. Since the tissue density between the intramuscular, subcutaneous, and intradermal tissue layers varies, the ease with which fluid may be injected into each type of tissue layer varies. The variation in tissue density causes changes in the backpressure exerted by the tissue against the fluid when it is injected. For instance, the backpressure associated with the intradermal tissue layer is greater than the backpressure associated with the subcutaneous tissue layer, thereby requiring a higher pressure and a greater force to accomplish the injection.
Currently, several pen injection systems are commercially available for subcutaneous substance delivery of medication. These pen injection systems typically use 29 to 31 gauge needles having lengths of between 5 mm and 12.7 mm, and are used to deliver the contents of a medicament cartridge, such as insulin, to the subcutaneous tissue layers of a patient rapidly and conveniently. The medicament cartridges are generally of a standard volume and size (including a fixed cross sectional area). The pressure of delivery is the quotient of the actuation force exerted by a user and the cross sectional area of the cartridge. Since the cross-sectional area of the cartridge is fixed, higher delivery pressures require higher actuation forces by the user.
A “microneedle” pen system has been developed to facilitate subcutaneous substance delivery. Such “microneedle” drug delivery systems may include shorter needles, typically less than or equal to 3 mm, with smaller diameters, in the range of 30 to 34 gauge or thinner. Such needle length and gauge size combinations are desirable to provide for sharp, yet short, point geometries that can more accurately target substance delivery to only certain selected tissue, such as the deep intradermal or shallow subcutaneous tissue layers, thereby permitting controlled fluid delivery. Current typical pen injection systems used for subcutaneous delivery are not believed optimal for use by the general population of self-injectors for delivery into the intradermal layer because of, inter alfa, the high backpressures associated with injecting fluid into the intradermal layers of the skin using microneedles.
To achieve effective medication delivery to the targeted tissue layer in light of higher backpressures, it is desirable to control two factors: the depth accuracy of the injection and the rate of the injection. This is of particular interest in connection with intradermal injections because the backpressures are relatively high, but similar analysis can be applied when injecting into the intramuscular or the subcutaneous tissue layers. The delivery of medicament within the narrow depth range of the intradermal tissue layer should first be assured, and maintained during injection. Once the depth accuracy is obtained, the rate of injection should be controlled to minimize or eliminate leakage of the medicament into other tissue layers or back out through the skin. Additional details of intradermal drug delivery and microneedles have been previously described in U.S. Pat. No. 6,494,865, issued on Dec. 17, 2002, U.S. Pat. No. 6,569,143, issued on May 27, 2003, PCT Publication No. WO2005025641, published Mar. 24, 2005, and U.S. Patent Application Publication No. 2005/0065472, published on Mar. 24, 2005, all of which are assigned to Becton, Dickinson and Company, and the entire content of each such patent and application being incorporated herein by reference.
The intradermal tissue layer of the skin is considerably denser than the subcutaneous tissue region. The density of the intradermal tissue layer on a particular patient is, in part, a function of their collagen make-up, which is affected by the patient's age, and the location of the injection site on the patient's body. This increased density of the intradermal tissue layer can create a greater backpressure resistance on the injection device than the resistance created when injecting into the subcutaneous tissue region. To overcome the increased backpressure resistance when injecting into the intradermal tissue layer with a conventional drug delivery pen, the user or patient would need to exert greater actuation force (which could be substantial) on the injector device actuator or employ some sort of powered injector device. In these applications, the injector device must be designed to withstand the greater backpressure from the intradermal injection site as well as the additional force exerted by the user or patient. Further, the increased actuation force required to actuate the injector device may result in the fluid “jetting” past the desired tissue depth due to the increased fluid pressure.
Conventional drug delivery pens may require that the user keep the needle seated in the skin for a period of up to about 10 seconds, after the injection has been completed, to allow for the “axial compliance” of the pen mechanism (or lead screw) and the cartridge back-end stopper to equilibrate to minimize “drool” from the needle tip upon withdrawal. Such time periods may need to be increased to accommodate any additional axial compliance resulting from higher backpressures, and such increased time periods can also decrease the required force to make the injection.
As advances in understanding the delivery of drug proceeds, the use of intradermal delivery systems is expected to increase. Use of a “standard” length needle to deliver a drug substance intradermally has its shortcomings, as noted above. It is not possible to use a delivery device having a needle length suited for intradermal injection to aspirate a syringe with drug substance from a multi-use vial. Thus, there are shortcomings in the prior art that prevent administering an intradermal injection using a “standard” length needle and a multi-use vial. It would be advantageous to have a drug delivery device capable of accessing substances stored in multi-dose vials and delivering such substances into the intradermal region of the skin without encountering the shortcomings described above.
Existing drug delivery pens offer several advantages over syringe based systems for delivering insulin subcutaneously. Reusable drug delivery pens hold 20 or more doses without requiring the drug cartridge to be refilled. Dose setting is achieved simply with the use of a dial. However, those drug delivery pens are designed for low pressure subcutaneous injections. Intradermal injection of insulin and other medications provides faster uptake of the drug, thereby leading to improved therapy. Existing drug delivery pens have several limitations regarding intradermal drug delivery. First, the mechanical advantage provided by the pen is minimal and requires the user to supply upwards of 20 lbs of force to generate sufficient pressure. Secondly, the pen components are often damaged by this high force, resulting in leaking and inaccuracy at the high pressures. Additionally, the size of the drug delivery pen required to obtain the high pressures associated with intradermal drug delivery would be too large for a user to conveniently carry.
There are no existing intradermal pen-like devices that take advantage of pen-like, dial-a-dose accuracy and ease of use with syringe like (small diameter) high pressure performance. Existing drug delivery pens require a large force to inject medication into the intradermal layer, thereby making the intradermal medication injection difficult. Furthermore, the drug delivery pen components are often damaged due to the high pressures, thereby resulting in medication leakage and dose inaccuracy.
Therefore, a need exists to provide a system and method for enabling users or patients to perform high pressure delivery of compounds, such as therapeutic drugs, vaccines, and diagnostic materials, at a controlled rate without requiring the exertion of an overly large force or resulting in an unwieldy device.